1. Field of the Invention
The present invention is directed to a second time around clutter cancellation system, and, more particularly, to a system that weights return signals from eight different radar pulses from three different pulse repetition frequency pulse groups and accumulates the weighted pulses as the filtered output.
2. Background of the Invention
In radar systems such as an air traffic control radar, ground clutter needs to be eliminated to provide a controller with an accurate picture of the location of various air vehicles. When a radar pulse reflects off an airplane and returns to a radar antenna, the airplane reflection can be accompanied by ground clutter reflections from objects such as large buildings and mountains located within the maximum range of the radar. The ground clutter (first time around clutter) within the maximum range of the radar can be removed by subtracting the return signal of a first pulse from the return signal of a second pulse. The resulting signal generally only includes objects which are in motion and the doppler frequency of the return indicates target velocity. However, when certain atmospheric conditions exist or when the area outside the maximum radar range includes large signal reflectors such as mountains, a reflection from the large reflectors produced by a much earlier pulse can be received. This unexpected return is interpreted as a moving object because there is not a corresponding inverse signal to cancel out the reflection. Such out-of-range reflections are called second time around clutter. Typical first time around clutter cancellation systems simply combine all the pulses with a pulse group, called a pulse repetition frequency group in which each of the transmitted pulses in the group are separated by the same time period or interpulse period, to produce a filtered return by subtracting half the pulses from the other half of the pulses. Even when all the pulses within the group are combined, the second time around clutter can still exist because of late arriving reflections from a previous pulse group. Since second time around clutter will be interpreted as moving objects within radar range, and because of the increased traffic load managed by controllers, it has become necessary to remove the second time around clutter.